A thermal head is used for recording with joule heat generated by exothermic resistors disposed so as to form lines separated by equal spaces. Recording methods for the thermal head are classified into two major categories: thermosensitive recording method and thermal transfer recording method. Thermal transfer recording method is further divided into melting type thermal transfer recording method and sublimation type thermal transfer recording method.
Thermosensitive recording method performs recording in the following manner. First, as illustrated in FIG. 21, thermosensible paper 105 is inserted between a platen roller 100 and a thermal head 102 provided with exothermic resistors 101. The thermosensible paper 105 is composed of a colour developing layer 103 and base paper 104. Next, an electric current is supplied to the exothermic resistors 101 of the thermal head 102 so that the exothermic resistors 101 generate joule heat. Then, the colour developing layer 103 develops colour with the joule heat, which in turn forms a visual image on the thermosensible paper 105.
Additionally, in order to fix the visual image, the thermosensible paper 105 may be exposed to ultraviolet ray radiation after being heated in the above manner. This method is called fixing type thermosensitive recording method.
Melting type thermal transfer recording method performs recording in the following manner. First, as illustrated in FIG. 22, transfer film 106 and recording paper 107 (ordinary paper) are inserted between a platen roller 100 and a thermal head 102 provided with exothermic resistors 101. The transfer film 106 is composed of base film 108 and heat melting ink 109 of a relatively low melting point.
Next, an electric current is supplied to the exothermic resistors 101 of the thermal head 102 so that the exothermic resistors 101 generate joule heat, which then heats the transfer film 106. The heat melting ink 109 on the transfer film 106 is thermally transferred to recording paper 107 to form a visual image thereon.
Sublimation type thermal transfer recording method performs recording in the following manner. First, as illustrated in FIG. 23, transfer film 110 and image receiving paper 113 are inserted at the same time between a platen roller 100 and a thermal head 102 provided with exothermic resistors 101. The transfer film 110 is composed of base film 111 and sublimational ink 112. The image receiving paper 113 is composed of dyestuff fixing layer 114 and synthetic paper 115.
Next, an electric current is supplied to the exothermic resistors 101 of the thermal head 102 so that the exothermic resistors 101 generate joule heat, which then heats the transfer film 110. The sublimational ink 112 of the transfer film 110 is thus sublimated. The sublimated sublimational ink 112 is thermally transferred to the dyestuff fixing layer 114 of the image receiving paper 113 to form a visual image thereon.
In FIGS. 21 through 23, the thermal head 102 is shown to be separated from the thermosensible paper 105, the transfer film 106 or the transfer film 110 for convenience in understanding the description. However, the actual thermal head 102 is pressed against the platen roller 100 at a predetermined force via the thermosensible paper 105, the transfer film 106 or the transfer film 110.
FIG. 24 is a simplified structural view of the conventional thermal head 102 employed in the above mentioned methods. The thermal head 102 is provided with a ceramic substrate 123 as shown in a enlarged view of a portion of the upper surface of a main body 122 including a heat radiating plate 121. A glass glaze layer 124 is provided on the ceramic substrate 123. The exothermic resistors 101 are provided on the glaze layer 124 so as to form lines separated by equal spaces.
A selectable electrode 125 is provided on an end of each exothermic resistor 101, whereas a common electrode 126 is provided on the other ends of the exothermic resistors 101. Therefore, any desired exothermic resistor 101 on the glaze layer 124 can be selectively heated when an electric current is supplied to the corresponding selectable electrode 125. The whole surface of the conventional thermal head 102 having such a basic structure is covered with an abrasion protection layer 127.
Ensuring such a thermal head 102 to be in a solid contact with the thermosensible paper 105, the transfer film 106 or the transfer film 110 is essential for high quality printing using the thermal head 102. The glaze layer 124 serves to improve the adhesion between the thermal head 102 and the thermosensible paper 105, the transfer film 106 or the transfer film 110.
Incidentally, since information has an increasingly greater value and role in today's society, there are strong demands for a printer incorporating a thermal head which realizes printing of high resolution and high quality. In order to meet these demands, exothermic resistors provided to the thermal head need to have a smaller width, and besides, spaces between neighbouring exothermic resistors need to be reduced.
Nevertheless, in the conventional thermal head 102, the exothermic resistors 101 are fixed on the ceramic substrate 123 and covered with the abrasion protection layer 127. Therefore, when the neigbouring exothermic resistors 101 are heated at the same time, they thermally interfere each other as a result of heat conduction. Consequently the area having higher temperatures than a predetermined temperature for printing extends unnecessarily around the exothermic resistors 101, which leads to an increase of dot diameters in printing and thus a sharp decline in print quality.
Moreover, in the arrangement of the conventional thermal head 102, the glaze layer 124 and abrasion protection layer 127 leak the joule heat generated by the heated exothermic resistors 101. Therefore, the electric current needs to compensate for that heat loss and still heat the exothermic resistors 101 up to the predetermined temperature, causing greater power consumption.
Furthermore, in the arrangement of the conventional thermal head 102, the provision of the glaze layer 124 only allows a linear portion of the thermal head 102 to come into contact with the thermosensible paper 105, the transfer film 106 or the transfer film 110. Therefore the linear portion (contact portion) wears off easily due to a strong force applied thereto. The whole surface of the thermal head 102 needs to be covered with the abrasion protection layer 127 to eliminate such inconvenience. The provision of the abrasion protection layer 127 pushes up the production costs and creates an obstacle in making a smaller and lighter thermal head 102.